Method and apparatus for calcining materials



Oct. 15, 1946. w. E.'HESSMAN 2,409,205

METHOD AND APPARATUS FOR CALCINING MATERIALS Filed April 9, 1941 3 Sheets-Sheet l n I l I a w a;

02; Will,

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METHOD AND APPARATUS FOR CALCINING MATERIALS Filed April 9, 1941 s Sheets-Sheet 2 INVENTOR I w/u/A 5. H'fSMAA /"/a Z BY ATT RNEYS M 15, 1946. w. E. HESSMAN METHOD AND APPARATUS FOR CALCINING MATERIALS 3 Sheets-Sheet 3 Filed April 9, 1941 J WJHW a INVENTOR Patented Oct. 15, 1946 METHOD AND APPARATUS For; CALCINING MATERIALS 7 William E. Hessman, Luckey," 'O hio, assignor to National Gypsum Company, Bufialo, N. Y.

Application April 9, 1941, Serial No. 387,625

21 Claims. 1

This invention relates to means for and a method of calcining limestone to produce lime or for analogous industrial processes.

It is well known in the art to use a plurality of furnaces each of which is arranged to discharge its gaseous products of combustion directly into a common kiln. The present invention may be said to present two principal aspects of novelty. One is in the provision of a single furnace having a novel draft arrangement, in and of itself, and the other resides in correlating the draft arrangements in a group of furnaces serving a common kiln.

Having reference to the present invention insofar as it concerns the disclosure of a novel draft valve arrangement for a single furnace, my invention resides in alternately driving hot gaseous products of combustion upwardly out of the fuel bed of the fire in the furnace and then driving a blast of air across the combustion chamber over the fuel bed and toward the associated lime kiln to move the hot gaseous products into immediate and direct contact with the charge in the kiln. This mode of operation is repeated successively and alternately substantially throughout the period of operation of the furnace. The blasts are of relatively short duration and succeed each other at a rate calculated to produce pulsating surges of heat energy.

I have found that the wirling and surging action produced in this manner operates in a very substantial way to avoid the usual channeling and pocketing of hot gases through and in the charge of the kiln and results in a highly uniform calcination of the charge by virtue of a very greatly increased uniformity of heat distribution in the kiln.

The general scheme of operation set forth in the foregoing is preferably supplementedby providing automatic operation of natural draft valves associated with both the ash pit and the combustion chamber of a furnace. By such automatic operation I propose to arrange the natural draft valve leading to the ash pit to be automatically closed when a forced blast of air is directed. into the ash pit and open when a forced blast is directed over the fuel bed in the combustion chamber.

Similarly, but conversely, I propose to provide a natural draft valve communicating directly with the combustion chamber above the fuel bed which is automatically closed when a forced blast of air is driven across the top of the fuel bed and automatically open when the ash pit forced blast is applied.

5 v Fig. l is a fragmentary elevational View of a normally arranged to discharge their gaseous products of combustion into the kiln at points spaced thereabout. I provide means whereby the operation of a number of furnaces related to one kiln operate successively with respect to their forced blast impulses so that the inrushing impulse of hot gaseous products to the kiln from the several furnaces is uniformly staggered as to time.

This arrangement is found to still further greatly improve the uniformity of distribution of heat in the kiln with a consequent greatly improved calcination of the charge therein. The swirling and surging of the variously directed pulsating impulses of hot gases in the kiln serves to substantially prevent channeling of the heat through the kiln and pocketing of the heat therein. An important end which is served by operation according to my invention is the driving off of the relatively heavy carbon dioxide gases from the limestone in the kiln and a forcible ejection of such gases at the top thereof.

The pulsating impulses of the hot gases from the several furnaces produce a buifeting about of the hot gases through the mass of limestone. The relatively uniform distribution of heat in the kiln permits complete calcination of the charge at less extreme maximum temperatures than heretofore with a consequently greatly increased life of the kiln lining. As previously pursued, conventional lime kiln methods have resulted in the necessity of rather frequent repair of the kiln lining due principally to pocketing of heat locally in the charge adjacent the kiln lining.

While a single specific embodiment of apparatus capable of achieving the foregoing objectives is illustrated in the accompanying drawings and described in detail below, it is to be understood that the operating principles herein contemplated may be followed in a number of ways and with considerable variation in the specific apparatus employed. Accordingly, my invention is not to be considered as limited otherwise than as defined in the appended claims.

In the drawings:

Fig. 4 is a diagrammatic horizontal cross sec-.

tional view of the entire kiln showing schematically the draft conduits of the several furnaces;

Fig. 5 is a vertical cross sectional view taken; through the master control valve for the forced draft;

Fig. 6 is a horizontal cross sectional view taken on the line VI-VI of Fig. 5; and

Fig. 7 is a valve diagram showing one complete cycle of operation of the drafts for the several related furnaces of the lime kiln of Figs. 1

through 4;

Figure 8 comprises an enlargement of the portion of the linkage appearing at the upper right hand corner of Figure 1.

Throughout the several figures of the drawings like characters of reference denote like parts and the numeral l0 designates a conventional lime kiln of the type comprising a vertical tower wherein the limestone charge is fed into the top of th kiln (not shown) and the calcined lime (calcium oxide), is removed at the bottom of the kiln, likewise not shown in the drawings. The present invention deals primarily with the means for firing the kiln which in the illustrated instance resides in the provision of a plurality of furnaces disposed about the kiln toward the bottom thereof and consequently only that portion of the kiln tower is illustrated. The kiln charging and discharging means may be selected from such means as are presently known for these purposes without affecting the application of the principles of my present invention to an otherwise conventional lime kiln.

In the present instance there are four furnaces associated with the kiln tower H] and for convenience of reference these furnaces have been numbered I through 4 as shown in the diagrammatic view, Fig. 4. These furnaces have been shown as being of the conventional dutch oven type excepting insofar as their novel shaft arrangement and operation are concerned.

In Fig. 1 furnace No. l is shown in vertical cross section and furnace No. 3 appears in elevation. In the ensuing discussion corresponding parts of the several furnaces and their appurtenances will be given like characters of reference, regardless of the fact that some of those parts are in different positions in their cycle of operation on the different furnaces at any given time.

In the form illustrated each furnace comprises a casing l2 which may comprise a fixed part of the structure of the kiln tower Ill. Within each furnace casing it there is provided a grate l3 with a conventional combustion chamber l4 above the grate and a conventonal ash pit l5 therebelow. A pair of fines is conduct the gaseous products of combustion from each furnace into the interior of the tower H] where they come into contact with the charge of limestone or other material to be calcined.

Each furnace unit, as appears best in Fig. 2, is provided with a conventional fire door It and an ash pit door 19. The ash pit door l9 may be provided with a natural draft valve as for in- 4 stance a flap 26 in the illustrated form. In proceeding in accordance with the present invention it is further desired to provide for the introduction of natural draft over the fire and to this end the illustrated embodiment is provided with a pair of sliding valves or gates 2| for each furnace unit. The natural draft valves is and 2| are arranged for automatic operation in a manner which will presently appear.

In addition to the natural draft valves referred to each furnace unit is provided with a pair of forced draft ducts, one above and one below the fire in each case.

In Fig. 2 the numeral 23 designates a forced draft duct below the grate and the numeral 2 designates a forced draft duct leading into the combustion chamber above the fire. The disposition and direction of the ducts 23 and 2!; appear further in the composite cross sectional view, Fig. 3 of the drawings, as directed obliquely toward the vertical center line of the ash pit and the combustion chamber respectively. Each of the forced draft ducts 23 and 24 are provided with control valves and such valves, in the form of sliding plates, are designated 2'! and 28 respectively.

Suitable mechanical instrumentalities and drive means are provided for opening and closing the several natural and forced draft valves in a particular sequence and for directing pressure blasts into the several combustion chambers and ash pits at predetermined related times in the course of operation of my improved lime kiln. Such mechanical means may be varied from those here shown without affecting the principles of my invention and at this point in the description of a single exemplary embodiment of my invention I shall describe the functional sequence of operation of the several valves and the pressure blasts without reference to the mechanical means for producing and controlling such functional sequence.

Reference is had to the composite valve diagram which is Fig. 7 and which shows the timed relationship of the draft valves and pressurq blasts of the four furnaces which are included in the present illustrative embodiment of the invention. It is to be noted that the several furnaces all discharge their gaseous products of combustion into the single kiln tower ID at substantially the same level but at points spaced about such tower and an object of the arrangement of the present invention resides in so correlating the operation of the several furnaces as to produce a uniform application of heat to the charge in the kiln to produce satisfactory calcination thereof as set forth in the remarks preliminary to the description of the present specific example.

The valve diagram, Fig. 7, shows a single complete cycle of operation of the four interrelated furnaces, and such cycle is represented as comprising 360. At the conclusion of the cycle shown, the several draft valves and the forced draft apparatus all are in the same condition and position in which they were at the beginning of such cycle.

Reference will first be had to the cycle of operation of a single furnace and for this purpose reference will be had to furnace No. I. It is noted that the cycles of the several furnaces are identical in their details excepting only for the fact that they are stepped as to phase of operation.

At the beginning of a complete cycle of operation, the lower forced draft valve 21 of furnace No. I has just opened and the upper forced draft valve 28 has just closed. At the same time the lower natural draft valve has just closed and the upper natural draft valves 2! have just opened.

This condition, in which the lower forced draft valve 27 and the upper natural draft valves 2! are open and the lower natural draft valve and the upper forced draft valve are closed, is indicated in the valve diagram by horizontal lining, and it will be noted that this condition prevails, in the case of furnace No. I, from the beginning of the cycle to a point half way through. At this latter point the conditions of the several valves of this furnace reverse; that is, the lower natural draft valves 29 and the upper forced draft valves 28 open and the lower forced draft valves 5' and the upper natural draft valves 2| close. This condition of the valves is indicated by vertical lining in Fig. 7 and it is noted that such condition prevails throughout the last half of the complete cycle in the case of furnace No. I.

While the mere opening of either the lower or upper natural draft valves and 2|, respectively, is self-sufficient to give access to draft air, the mere opening of either the lower or upper forced draft valves 27 and 28, respectively, is merely a condition precedent to the actual application of a forced blast of draft air. Such blast of draft air is, in the present instance, of relatively short duration and it is only required that the forced blast be applied to the duct 23 for a brief period during the 180 portion of the cycle that the lower forced draft valve 21 is open, and, further, that a second forced draft blast be applied to the upper forced draft duct 24 for a brief period during the 180 interval when the upper forced draft valve 28 is open.

Only a single forced draft source is utilized in the apparatus which forms the present illustration and that single source is variously diverted during the complete cycle of operation by suitable valve mechanism which will presently be described. The braces which extend from the 45 to the 90 point in the case of furnace No. I, and likewise from the 225 to the 270 point, indicate that during these two different 45 intervals, forced draft is applied.

The ducts 23 and 24 are branches of a common supply duct 33 which is indicated schematically in Fig. 4. By virtue of this arrangement it is only necessary for the master controlling valve mechanism, indicated schematically at 3!, to clivert pressure blasts to the four furnaces by means of four discharge ducts, one for each furnace. The ultimate diversion of such forced blast to either the ash pit or the combustion chamber of the furnace is controlled solely by the forced draft valves 21 and 28 at the furnace. It is sufficient for the present to state that the master valve mechanism 3! is arranged in such manner that it directs two forced blasts into the duct 3i! directed to furnace No. l during each complete cycle of operation and these two blasts occur as indicated by the braces in the valve diagram.

The practical result of this draft valve cycle is that a blast of air is forced under the grate, with the natural draft valve for the ash pit in a closed condition to prevent the escape of air, and to force such blast through the fuel bed. At the same time the upper natural draft valve is open to admit additional draft air above the fuel bed.

Following this, the lower natural draft valve opens and a forced blast is-forced overthe fire to move the products of combustion into thekiln. Again to prevent the escape or diversion of the forced blast, the upper natural draft valvesare closed at this time. J

The pulsating or surging action so'pmduced has the effect of causing the hot gases and the flame to whirl and surge through the mass of limestone in the kiln and is found to avoid channeling and pocketing of the heat in -a hitherto unattainable manner. The draft cycle hereinbefore disclosed is further found to be unusually efiicacious in forcing the carbon dioxide gases evolved in calcination to the thrown off from the limestone and out of the top of the kiln.

The operation of each of the three remaining furnaces is identical with that of furnace No. with two exceptions only. The first is unimportant and resides in the fact that blasts indicated by the braces in Fig. 7 vary slightly asto their time of occurrence in the several instances. This is merely a question of expedience in arranging the valve control mechanism and may be dispensed with by employing a somewhat more elaborate valving arrangement.

The second difference as between the several furnaces is one which further promotes uniformity in the application of the hot gaseous products of combustion about the kiln, and further augments the surging and whirling action ofsuch gases to further enhance the efficiency and uniformity of operation of my new kiln. Each of the furnaces from I through 4 is behind the preceding furnace in phase so that the peak discharges of hot gaseous products from the several furnaces follow each other at regularly timed intervals. It is believed that this recurring phase relationship is clearly evident from a consideration of the valve diagram, Fig. '7.

Reference will now be had to a master control valve means for producing appropriately timed blast impulses directed successively to the several furnaces. Such means is illustrated in detail in Figs. 5 and 6 and schematically at 3| in Fig. 4.

Reference has previously been had to a common forced blast duct 38 which supplies the lower and upper forced draft ducts 23 and 24 of furnace No. I. A similar common blast supply duct 32 is provided for the upper and lower forced blast ducts of furnace No. 2, and similar ducts 33 and 34 perform a like function for furnaces Nos. 3 and 4 respectively.

Referring to the diagrammatic showing of Fig. 4, the numeral 37 designates a blower or other suitable air pressure generator means adapted to be driven by a motor 38 or the like. The blower draws its air from heat exchange coils 39 arranged adjacent the exteriors of the several furnaces whereby the forced blast air is pre-heated to some extent. The blower directs its output air to a cylindrical casing 40, which forms the main chamber of the master control valve means designated generally at 3i, by means of a conduit 4| which discharges into the casing 49. An end wall 42 of the casing 40 has four ports spaced equally about a common circle struck from the center of the end wall 42 and one each of these ports communicates with each of the ducts 3!], 32, 33 and 34, which lead to furnaces Nos. I through 4, respectively.

A rotatable valve plate 43 is disposed against the inside of the end wall 42 and is provided with an arcuate port 45 which lies along an arc coincident with the circle about which the ports leading to the ducts 30, 32, 33, 34 are spaced. The valve plate 43 serves to close all of the duct pasthe forced draft sages excepting the one which happens to be in registry with the port of the valve plate.

By rotation of the valve plate 43 the ducts 30, 32, 33 and 34 are successively placed in communication with the interior of the casing 43 and thereby with the supply duct 4|. Since two pressure blasts are desired to be delivered to each furnace during a 360 cycle of operation, the valve plate 43 is arranged to rotate twice during each such cycle. Rotational impulses are delivered thereto by means of a central shaft 41 extending through and journaled in a cylindrical casing 40 and bevel gears 48 which connect the valve shaft 41 with a drive shaft 49.

The sequence in which the ducts for the four furnaces receive blasts from the casing 49 by rotation of the valve plate 43 is as follows. First, furnace No. I; second, furnace No. 3; third, furnace No. 2; and fourth, furnace No. 4. The

manner in which this sequence of blasts ties in with the sequential operation of the four furnaces will be clear from a study of Fig. 7 and particularly the braces in that figure which indicate the application of pressure blasts.

Any convenient means for opening and closing the various forced and natural draft passages may be availed of as long as the operation of such means results in timing of the opening and closing movements in accordance with the principles laid down in the foregoing discussion. As one example of suitable valve opening and closing means I have shown associated with each furnace a rock shaft 50 adapted to be oscillated and, by suitable connection to the several valves, to transmit such oscillations to the latter to open and close the same in opposite directions of oscillation. The rock shaft may be journaled in bearings 5| secured against the front wall of each furnace and each rock shaft 5|] is caused to oscillate by connection with a continuously rotating shaft 53 which, in the present instance, is carried in bearings secured against the tops of the several furnaces.

In the present instance a single shaft 53 at each side of the kiln serves to oscillate the pair of rock shafts 59 for the two furnaces at that side of the kiln. Each rock shaft 50 has a relatively short driving arm 55 which has connection with a crank 56 on the shaft 53 by means of a link 51. Connection between the link 51 and the driving arm 55 of the rock shaft 59 is by means of an elongate slot 58 in the link engaged by a pin 59 carried by the outer end of the arm 55. In this way, while the crank 56 rotates continuously, it

transmits rocking movement in opposite directions to the driving arm 55 intermittently; namely, only when an end of the slot engages the'driving arm pin. By virtue of this construction the opening and closing of the several valves at the furnace by oscillation of the rock shaft is localized so as to occur in a fairly brief changeover period.

The connections between the four rock shafts 50 and their associated furnaces are the same in the case of the several furnaces and consequently, reference will be had by way of example only to furnace No. I, which appears at the righthand portion of Figs. 1, 2 and 3, respectively. An enlargement of the portion of the linkage appearing at the upper right hand corner of Figure 1 is shown in Figure 8.

In the case of furnace No. a, pair of rock arms 30 are carried by the shaft 59 and connected at their outer ends with the upper natural draft valves 2| by chains or the like, 6|. The lower natural draft valve 20 likewise is controlled by a rock arm 52 carried by the shaft 50, but in this instance an intermediate rock shaft 63 is journaled against the front of the furnace and has a rock arm 64 which is connected with the arm 62 by a chain 65 or the like for receiving its rocking impulses. The intermediat rock shaft 63 has a second rock arm 61 which engages one end of a link 68 whose other end is pivotally connected to the draft valve 20 to raise and lower the same about its upper hinged connection. This indirect connection between the rock arm 62 and the draft valve 20 is provided merely to avoid interference with the door |8 leading to the combustion chamber.

It will be noted that the rock arms 60 and the rock arms 62 extend in opposite directions from the shaft 50 so that movement of one of them to close its associated natural draft valve is accompanied by movement of the other rock shaft to open its associated natural draft valve.

Each rock shaft 55 is also provided with a rock arm 19 which is connected with both the upper and lower forced draft valves 28 and 21, respectively, by chains 1|, E2 or the like. Ihe valves 21 and 28 are accordingly arranged to move upwardly and downwardly in unison with the upper natural draft valves 2|. It will be noted, however, that the lower forced draft valve 2! is closed by downward movement and opened by upward movement while the reverse is true in thecase of the upper forced draft valve 28. For this reason closing movement of the upper natural draft valves 2| is accompanied by opening movement of the upper forced draft valve 28 and closing movement of the lower forced draft valve 21.

It will, of course, be understood that the rocking impulses transmitted to the rock shaft 50 of each of the four furnaces by means of a crank 55, link 5'! and rock arm 55, will be 90 behind in operation with respect to the preceding furnace, having in mind the valve diagram, Fig. '7, and the successive operations there indicated.

To produce the same relative direction of rotation at opposite sides of the kiln the shaft 53 for furnaces Nos. and 2, and the shaft 53 for furnaces Nos. 3 and 4, must rotate in opposite directions, and such opposite rotation may conveniently be attained by providing a pair of intermediate shafts extending coaxially and at right angles to the shaft 53. The shafts each engage a shaft 53 by virtue of a beveled gear connection 8|, and a common bevel driving gear 82 carried by a drive shaft 83 engages a pair of bevel gears 84, one on each of the shafts 80, to rotate the two shafts 85' in opposite directions.

Attention has previously been called to the fact that the master control valve 43 is desired to be driven at twice the angular velocity of the shafts 53 which control the draft valves for the individual furnaces. This ratio may be established in any convenient manner and I have indicated schematically in Fig. 4 a speed reducing unit 88 which is caused to rotate both shafts 49 and 83, for the master valve and the individual furnace valves, respectively, by means of a driving motor 89 or the like.

While variations in the speed of operation of my improved pulsating draft arrangement may be made without departing from the teachings hereof, I have found that operation of the apparatus in such manner as to produce thirty-two blast impulses per minute, eight at each furnace, four above and four below the grate, operates in a highly satisfactory manner,

What is claimed is:

1. calcining apparatus comprising a charge receiving chamber, a furnace associated therewith, a grate in said furnace with a combustion chamber above the grate and an ash pit therebelow, a. conduit between said combustion chamber and said charge receiving chamber whereby gaseous products of combustion from said combustion chamber may be discharged into said charge chamber, draft means associated with said ash pit and other draft means associated directly with said combustion chamber, and automatic means for rendering both draft means intermittently operative in substantially continuously recurring succession. a

2. Calcining apparatus comprising a charge receiving chamber, a. furnace associated therewith, a grate in said furnace with a combustion chamber above the grate and an ash pit therebelow, a conduit between said combustion chamber and said charge receiving chamber whereby gaseous products of combustion from said combustion chamber may be discharged into said charge chamber, draft means associated with said ash pit and other draft means associated directly with said combustion chamber, and automatic means for alternately rendering said ash pit draft means operative and inoperative in sub stantially continuously recurring succession and for simultaneously rendering the combustion chamber draft means operative when the ash pit draft means is inoperative and vice versa.

3. Calcining apparatus comprising a charge re,- ceiving chamber, a furnace associated therewith, a grate in said furnace with a combustion cham ber above the grate and an ash pit therebelow,

a conduit between said combustion chamberand said charge receiving chamber whereby gaseous products of combustion from said combustion chamber may be discharged into said charge chamber, forced draft means associated with said ash pit and other forced draft means associated directly with said combustion chamber, a natural draft valve for said ash pit and a second natural draft valve for said combustion chamber, and automatic operating means whereby the natural draft valves are opened and closed successively, the natural draft valve for the combustion chamber being closed when the natural draft valve for the ash pit is open and vice versa, and means for directing forced draft to said ash pit forced draft means while the ash pit natural draft valve is closed and for directing forced draft to said combustion chamber forced draft means while the combustion chamber natural draft valve is closed.

4. Calcining apparatus comprising a charge receiving chamber, a furnace associated therewith, a grate in said furnace with a combustionchamber above the grate and an ash pit therebelow, a conduit between said combustion chamber and said charge receiving chamber whereby gaseous products of combustion from said combustion chamber may be discharged into said charge chamber, forced draft means associated with said ash pit and other forced draft means associated directly with said combustion chamber, and automatic control means for both of said forced draft means for alternately directing forced blasts of draft air to said ash pit and to said combustion chamber in substantially continu-' ously recurring succession.

5. Calcining apparatus comprising a charge receiving chamber, a furnace associated therewith, a grate in said furnace with a combustion .when said natural draft valve is closed.

6. Calcining apparatus comprising a charge receiving chamber a furnace associated therewith, a grate in said furnace with a combustion chamber above the grate and an ash pit therebelow, a conduit between said combustion chamber and said charge receiving chamber whereby gaseous products of combustion from said combustion chamber may be discharged into said charge chamber, a natural draft valve communicating directly with said combustion chamber, automatic means for. opening and closing said nat ural draft valve in substantially continuously recurring succession, means for directing forced blasts of air into said combustion chamber, and means for timing such blasts to occur during-periods when said natural draft valve is closed.

7. Calcining apparatus comprising a charge receiving chamber, a furnace associated therewith, a grate in said furnace with a combustion cham-I ber above the grate and an ash pitthereb'elow, a conduit between said combustion chamber. and said charge receiving chamber whereby gaseous products of combustion from said combustion chamber may be discharged into said charge chamber, forced. draft means associated with said ash pit and other forced draft means associated with said combustion chamber, and automatic control means for both of said forced draft means for alternately directing forced blasts of draft air to said ash pit and to said combustion chamber in substantially continuously recurring succession, the combustion chamber draft means being arranged to direct the forced blast'over a fuel bed and. toward the conduit leading tothe charge receiving chamber. a i

8. Calcining apparatus comprising a charge re ceiving chamber, a furnace associated therewith, a, grate in said furnace with a combustion chamber above the grate and an ash pit therebelow, a conduit between said combustion chamber and said charge receiving chamber whereby gaseous products of combustion from said combustion chamber may be. discharged into said charge chamber, a. natural draft valve associated with said ash pit and a second natural draft valve associated directly with said combustion chamber, automatic means for simultaneously opening one of said valves and closing the other in substantially continuously recurring succession. l

9. Calcining apparatus comprising a charge receiving chamber, a plurality of furnaces associated therewith each having a grate with acornbustion chamber thereabove and an ash pit there below, a conduit between each furnace combustion chamber and said charge receiving chamber whereby the gaseous products of combustionfrom said furnacesare discharged into said' charge chamber, each furnace having draft means associated with the ash pit thereof and other draft means associated directly with its combustion chamber, the draft means of each furnace being adapted to be rendered intermittently operative 11 in substantially continuously recurring succession, and means synchronizing the draft means operation of the severalf-urnaces, whereby to operate successively in a continuously recurring cycle.

10. Calcining apparatus comprising a charge receiving chamber, a plurality of furnaces associated therewith each having a grate with a combustion chamber thereabove and an ash pit there,- below, a, conduit between each furnace combustion chamber and said charge receiving chamber whereby the gaseous products of combustion from said furnaces are discharged into said charge chamber, each furnace having draft means associated with its ash pit and other draft means associated directly with its combustion chamber, means associated with each furnace for alternately rendering the ash pit draft means operative and inoperative in substantially continuously recurring succession and for simultaneously rendering the combustion chamber draft means operative when the ash pit draft means is inoperative, and vice versa, and means synchronizing the draft means of the several furnaces whereby they operate successively in a continuously recurring cycle.

11. Calcining apparatus comprising a charge receiving chamber, a plurality of furnaces associated therewith each having a grate with a combustion chamber thereabove and an ash p t therebelow, a conduit between each furnace combustion chamber and said charge receiving chamber whereby the gaseous products of combustion from said furnaces are discharged into said charge chamber, forced draft means associated with the ash pit of each furnace and other forced draft means associated directly with its combustion chamber, control means for both of the forced draft means of each furnace for alternately directing forced blasts of draft air to its ash pit and to its combustion chamber in recurring succession, and means synchronizing the control means for the several furnaces whereby the phases of draft operation of the several furnaces are stepped uniformly to follow each other in continuously recurring succession.

12. Calcining apparatus comprising a charge receiving chamber, a plurality of furnaces associated therewith eachhaving a grate with a combustion chamber thereabove and an ash pit therebelow, a conduit between each furnace'combustion chamber and said charge receiving chamber whereby the gaseous products of combustion from, said furnaces are discharged into said charge chamber, each furnace having associated therewith means for intermittently applying thereto a forced blast of draft air, and means synchronizing the forced blasts of the several furnaces whereby they occur successively.

13. Calcining apparatus comprising a charge receiving chamber, a plurality of furnaces associated therewith each having a grate with a combustion chamber thereabove and an ash pit therebelow, a conduit between each furnace combustion chamber and said charge receiving chamber whereby the gaseous products of combustion from said furnaces are discharged into said charge chamber, each furnace having associated therewith means for intermittently directing a forced blast of air over the fuel bed and toward its charge chamber conduit, and means synchronizing the forced blasts of the several furnaces whereby they occur successively with respect to each other.

14. Calcining apparatus comprising a charge receiving chamber, a plurality of furnaces associated therewith each having a grate with a combustion chamber thereabove and an ash pit therebelow, a conduit between each furnace combustion chamber and said charge receiving chamber whereby the gaseous products of combustion from said furnaces are discharged into said charge chamber, each furnace having associated therewith means for intermittently applying a forced blast of draft air to its ash pit, and means synchronizing said forced blast means whereby the forcedv blasts of the several furnaces occur successively in a continuously recurring cycle.

15. Calcining apparatus comprising a charge receiving chamber, a plurality of furnaces associated therewith each having a grate with a combustion chamber thereabove and an ash pit therebelow, a conduit between each furnace combustion chamber and said charge receiving chamber whereby the gaseous products of combustion from said furnaces are discharged into said charge chamber, forced draft means associated with the ash pit of each furnace and other forced draft means associated directly with its combustion chamber, a natural draft valve for the ash pit of each furnace and a second natural draft valve for its combustion chamber, operating means whereby the natural draft valves are opened and closed successively, the natural draft valve for the combustion chamber being closed when the natural draft valve for the ash pit is open, and vice versa, means for directing forced draft to the ash pit forced draft means of each furnace while its ash pit natural draft valve is closed and for directing forced draft to its combustion chamber forced draft means while its combustion chamber natural draft valve is closed, and means synchronizing the operating means of each furnace to operate the draft mechanisms of the several furnaces so that their like phases of operation are stepped uniformly to occur successively at regular intervals in a continuously recurring cycle.

16. A method of calcining material in a kiln having associated therewith a furnace arranged to discharge its gaseous products of combustion into said kiln and comprising a combustion chamber, a fuel bed supporting grate and an ash pit; said method comprising successively and alternately directing forced blasts of air into said ash pit and into said combustion chamber respectively.

17. A method of calcining material in a kiln having associated therewith a furnace arranged to discharge its gaseous products of combustion into said kiln, and comprising a combustion chamber, a fuel bed supporting grate, and an ash pit; said method comprising first, directing a blast of air into said ash pit, then directing a blast. of air into said combustion chamber above the fuel bed and in the direction of discharge of gaseous products into said kiln, and successively ep at n s ch eq ence of bla s.

18. A method of calcining material in a kiln having associated therewith a furnace arranged to discharge its gaseous products of combustion into said kilnand comprising a combustion chamher. a fuel d s pp rt g rad and an ash pit; said method comprising successively and alternately directing forced blasts of air into said ash pit and into said combustion chamber respectively at a frequency of the order of one or more cycles per minute.

19 A method. of calcining material in a kiln having associated therewith a furnace arranged to discharge its gaseous products of combustion into said kiln and comprising a combustion chamber, a fuel bed supporting grate, and an ash pit; said method comprising first directing a blast of air into said ash pit, then directing a blast of air into said combustion chamber above the fuel bed and in the direction of discharge of gaseous products into said kiln, and successively repeating such sequence of blasts at a frequency of the order of one or more cycles per minute.

20. A method of calcining material in a kiln having associated therewith a plurality of furnaces arranged to discharge their gaseous products of combustion into said kiln and each comprising a combustion chamber, a fuel bed supporting grate and an ash pit; said method comprising successively and alternately directing forced blasts of air into the ash pit and into the combustion chamber respectively of each furnace and timing the application of such pair of forced blasts to each furnace to occur successively at 14 equally spaced intervals throughout a full cycle of operation.

21. A method of calcining material in a kiln having associated therewith a plurality of furnaces arranged to discharge their gaseous products of combustion into said kiln and each comprising a combustion chamber, a fuel bed supporting grate and an ash pit; said method comprising successively and alternately directing forced blasts of air into the ash pit and into the combustion chamber respectively of each furnace, the combustion chamber forced blast being effected above the fuel bed and in the direction of discharge of gaseous products into said kiln, and timing the application of such pair of forced blasts to each furnace to occur successively at equally spaced intervals throughout a full cycle of operation.

' WILLIAM E. HESSMAN. 

